The forward fan high bypass engine incorporates a gas producer unit which is furnished air from a compressor that is driven by a turbine. Extending forward of the compressor is a shaft which through reduction gearing drives a large diameter ducted fan. The fan rotates in an annular duct which is a continuation of the air intake. The airstream accelerated by the fan is divided. One part of the airstream (primary air) is fed to the compressor to provide an air supply for the gas producer unit. The other part of the airstream (secondary air) bypasses the gas producer through an annular duct which surrounds the turbine. In some turbofan engines this portion of the airstream is ejected directly as a "non-mixed" exhaust. In other engines the bypassing airstream is "mixed" with the gas-generator exhaust at the core engine jet nozzle. The object of the bypass system is to combine the good operating efficiency and high thrust capability of a turboprop and the high speed, high altitude capability of a turbojet while achieving lower fuel consumption.
This invention provides a means for reducing the thrust output of the bypassing portion of the engine airstream at low throttle settings. The requirement to reduce thrust materializes when an aircraft equipped with high bypass fan engine(s) is waiting in line on a taxi strip or airport apron. The thrust of a high bypass engine, even at idle power setting, can be sufficient to cause the aircraft to move forward unless the flight crew maintains constant brake pressure. This task, when the aircraft encounters long waiting periods for take-off clearance, increases crew fatigue and interferes with normal pre-flight check lists and communications.
A prior art system of engine thrust control is described on page 118 of R. A. Fry's book titled "The Principles and Construction of Aircraft Gas Turbines", published by Sir Isaac Pitman and Sons, Ltd., London, 1960. In the system described by Fry, there is a ring of non-rotating guide vanes placed in front of the mechanically driven fan. The angle of incidence of these guide vanes can be varied to suit changing operating conditions.
There is a drawback to this approach which my invention overcomes. In the system described by Fry, varying the pitch of the entry vanes changes the amount of air going to both the gas producer and the bypass ducts. With the airstream going to the gas producer choked down the pilot will have to change throttle settings to maintain efficient conditions in the turbine. In addition, the inlet vanes must be anti-iced, which adds considerable complexity to the engine system. Using my invention, the air intake of the gas producer section is not disturbed when the thrust spoilers are activated. Thus, no change in throttle setting of the engine is required.